This invention relates to a composition of a coating solution, a process for its preparation and a process for producing a layer of cadmium stannate on a substrate with the coating solution.
Cadmium stannate layers are known to have high electrical conductivity, high infra-red reflecting properties and a low absorption within the visible spectrum. These characteristics of cadmium stannate layers make them suitable for example, as layers in heated windows, windows for buildings, doors for microwave ovens or solar collector panels.
Conventionally cadmium stannate layers have been applied to substrates using high frequency atomisation. This process takes considerable time to deposit the desired thickness of film and usually requires expensive production equipment. Clearly when coating large or numerous substrates this is uneconomical.
An alternate process for producing a cadmium stannate layer upon a substrate has been proposed which entails spraying of an aqueous solution of cadmium and tin salts, to which other salts have been added as process additives, onto hot glass. The temperature of the glass (usually ranging from 600 to 700.degree. C.) is critical as CdSnO.sub.3 or Cd.sub.2 SnO.sub.4 as well as CdO and SnO.sub.2 may be formed. This process does not appear to achieve a uniform layer corresponding to the chemical composition of the coating solution.
A further approach is disclosed in U.S. Pat. No. 4,229,491 which describes a process in which glass is dipped at room temperature into a coating solution. Then it is withdrawn steadily from the coating solution and heated to a high temperature, to form a transparent cadmium stannate layer. This is said to ensure a fully homogenous cadmium stannate layer and allow economical coating of glass panes as well as tubes of any given shape.
The patent describes the use of alcoholic solutions containing hydrolysable complexes formed from cadmium and tin compounds as dipping solutions. These complexes are derived from cadmium (lower alkyl) carboxylate, e.g. cadmium acetate, and a tin alkoxide, e.g. tin tetra-n-butylate. The preferred Cd:Sn ratio is from 1:1 to 2:1. Whereas acetylacetone is disclosed as an additive to such dipping solutions its effect on the process is not discussed.
Detailed studies of the system described in the U.S. Pat. No. 4,229,491 have shown that alcoholic solutions of tin alkoxides and cadmium acetate in the presence of acetylacetone alone are unstable and allow precipitation to occur. Such solutions do not give reliable cadmium stannate films. The films produced by this process may easily become subject to severe crazing and thus liable to be removed from the substrate to which they have been applied.